Process and plant for metallization of cast-iron pipes

ABSTRACT

A plant for metallization of a metal pipe obtained by upward vertical continuous casting and displaced vertically in a continuous manner, includes, along the path of the pipe, a metallization station having a passage for the pipe, a metallization enclosure ( 6 ), spray guns ( 11 ) mounted inside the enclosure on an oscillating plate ( 10 ) surrounding the path, means for supplying each gun with metallization material ( 12 ), and a device for causing oscillation of the support plate and for displacing the guns inside the enclosure with an oscillating movement, the frequency and angular amplitude of which ensure the formation of a coating of uniform thickness.

BACKGROUND OF THE INVENTION

The present invention relates to a process for metallization of metalsubstrates and relates more particularly to the hot metallization ofpipes, in particular cast-iron pipes which are obtained by means ofvertical continuous casting.

Among the known processes for the hot metallization of pipes, thefollowing are known:

customized hot-galvanization;

the continuous coating of steel plates with Zn or ZnAl;

methods for thermal spraying with Zn or ZnAl alloys;

zinc-coating of pipes.

The existing solutions based on galvanization have the followingdrawbacks.

Customized hot-galvanization coats the pipe internally and externallyand the pipe undergoes an increase in temperature such that the resultsof other prior operations carried out on the pipe at lower temperaturesrisk being affected.

Hot galvanization by movement through a liquid poses the problem,currently unsolved, of tightness with respect to the liquid bath betweenthe vertical pipe and the liquid metal, as well as the problem ofmaintaining a bath which is clean (without matte) and has a stablecomposition over time.

Cold metallization associated with annealing would ensure gooddistribution if rigorous conditions for preparation of the pipe surfaceare satisfied and would probably require heat treatment in an inertatmosphere.

The cost of such a treatment cycle would be considerable and as a resultwould offer limited advantages from an economic point of view.

The pipe to be coated, emerging after continuous casting, is not rough,which makes fixing of a cold coating impossible without carrying outsurface preparation.

In order to obtain a well-distributed coating, a pipe without an oxideskin is necessary and the working temperature must be high or else thepipe handling time must be long.

SUMMARY OF THE INVENTION

The invention aims to provide a uniformly distributed coating which hasa good impact resistance.

Moreover an object of the invention is to provide a coating whichadheres very well to the outside of a pipe of varying position anddiameter, continuously moving in a vertical direction and withoutundergoing rotation, while preventing the pipe from being subject toheat annealing.

Furthermore an object of the invention is to provide a coating which is“self-healing” in respect of any damage suffered.

Finally, an object of the invention is to provide a coating whichcontributes to corrosion resistance in underground conditions.

The invention therefore relates to a process for metallization of ametal pipe obtained by vertical casting and displaced vertically in acontinuous manner with an upward movement, characterized in that itconsists in:

cooling the pipe in an atmosphere which is inert with respect tooxidation up to a temperature of 700 to 900° C.; and

spraying onto the pipe, cooled beforehand to the aforementionedtemperature, a zinc-based metallization alloy with the aid of a set ofspray guns arranged so as to surround the path of the pipe to bemetallized.

According to a particular characteristic feature of the invention, thespray guns are made to oscillate about the path of the pipe with afrequency and an angular amplitude adapted to the speed of displacementof the pipe with a view to obtaining a coating of uniform thickness.

The invention also relates to a plant for metallization of a metal pipeobtained by means of upward vertical continuous casting and displacedvertically in a continuous manner, characterized in that it comprises,along the path of the pipe, a metallization station comprising a passagefor the pipe, a metallization enclosure, spray guns mounted inside thesaid enclosure on an oscillating plate surrounding the said path, meansfor supplying each gun with metallization material and means for causingoscillation of the support plate and for displacing the guns inside thesaid enclosure with an oscillating movement, the frequency and angularamplitude of which ensure the formation of a coating of uniformthickness.

The invention relates moreover to a metal pipe obtained by means ofvertical casting, characterized in that it has a metallization coatingobtained by the process defined above.

According to another particular characteristic feature, the spray gunsare three in number and are arranged on the plate at 120° from oneanother.

According to another particular characteristic feature, the spray gunsare flame guns or arc guns.

According to another characteristic feature of the invention, themetallization material is a wire of Zn_(x)Al_(1-x) alloy and the meansfor supplying metallization material comprise for each gun a wire storeand an unwinding device operated by the oscillation movement of theplate supporting the guns.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be understood more fully upon reading of thedescription which follows, provided solely by way of example and withreference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic, vertically sectioned view of a metallizationplant according to the invention;

FIG. 2 is a diagrammatic view sectioned along the line 2—2 of FIG. 1;

FIG. 3 is a partial diagrammatic side view of the metallization plantaccording to FIG. 1;

FIG. 4 is a partially sectioned view of a flame gun of the plant; and

FIG. 5 is a micrograph of a coating obtained by the process according tothe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The metallization plant shown in FIG. 1 has, along the path of a pipe Tobtained in an upward vertical continuous casting plant (not shown) andabove this plant, a shaft 1 for passage of the pipe T beingmanufactured, into which there is injected, via a duct 2, nitrogen forallowing the pipe T to be displaced between he casting plant outlet andthe metallization plant in an atmosphere protecting the surface of thepipe T against oxidation.

The shaft 1 has a lower part 3 which the pipe gradually cools during itsupward movement from 1100° C. to 1000° C. and a water-cooled upper part4 situated immediately below the metallization zone and inside which thetemperature of the pipe cools from 1000° C. to a temperature of between700 and 900° C.

A sliding mask 5 made of heat-insulating material is arranged inside theshaft 1 at the level of the joint between the lower part 3 and the upperpart 4 of the latter and its movement inside the upper part 4 allowscooling of the pipe T to be controlled by means of positioning betweenthe pipe and the wall of the said cooled part.

Above the shaft 1 there is arranged a metallization enclosure 6comprising a bottom 7 inclined downwards, from the center towards theperiphery, and provided with suction ducts 8.

In its central part, the bottom 7 is connected to a drop preventioncover 9 through which the pipe T to be coated passes with a minimumamount of play so as to prevent the sprayed metallization product whichdoes not adhere to the pipe wall from falling towards the bottom of theplant.

The enclosure has moreover an oscillating plate 10 on which spray gunssuch as flame metallization guns 11 are mounted.

Each gun is supplied with metallization wire 12 in a manner which willbe described with reference to FIGS. 2 to 4.

The guns 11 are for example three in number and are arranged on theplate 10 at 120° from one another.

Each gun 10 sprays a jet 13 of metallization material onto the pipe Tduring its upward movement.

The spray guns may also be arc guns.

They may also consist of devices for atomization of liquid metal.

Flame guns, however, have the advantage that they constitute a heattechnology which does not disturb the cooling kinetics of the product.

They ensure an excellent performance with regard to the spraying of theatomized droplets of alloy because these droplets are not driven awayfrom the substrate as, for example, in the case where arc guns are used.

The flow of flame guns is finer than that of arc guns.

They require only one wire of metallization material, while two wiresare required for arc guns.

The upper wall 14 of the enclosure 6 also has suction ducts 15.

The enclosure 6 has mounted on it a water-cooled casing 16 allowing themetallized pipe to emerge form the metallization plant at a temperatureof 750° C. and forming a thermal screen which prevents the plant frombecoming overheated.

The plant shown in cross-section in FIG. 2 has a platform (not shown)through which the pipe being formed passes and on which themetallization chamber provided with the annular plate 10 is mounted.

The plate 10 is driven in rotation with an oscillating movement by asuitable mechanism, not shown.

As mentioned further above, in the present example, said plate carriesthree flame metallization guns 11.

This number of three ensures the best compromise between the cost of theplant and the uniformity of the thickness of the coating. However, thenumber of guns may be different from three.

The amplitude of angular displacement of the plate 10 as well as itsfrequency are a function of the angle of dispersion of the jets 13 ofmolten metal which are sprayed by the guns 11 as well as the speed ofdisplacement of the pipe T so as to allow application of a metallizationlayer of uniform thickness.

Each metallization gun 11 which has means for supplying oxygen and gasby means of suitable hoses (not shown) has associated with it a devicefor supplying metallization product in the form of wire 12.

Each device comprises a wire store 20 and an unwinding device denoted bythe general reference 21, intended to convey the metallization wire 12from the wire store 20 to the associated gun 11, taking account of theoscillating movements to which the gun 11 mounted on the oscillatingplate 10 is subjected.

The wire 12 advantageously consists of an alloy containing 5 to 15% ofAl.

It is also possible to use a lined wire consisting of a Zn coresurrounded by an Al sheathing, in which case, after melting, a Zn₄₅Al₅₅alloy may be obtained. The lined wire may also consist of an aluminumcore and a Zn sheathing.

The best lined wire or the best alloy is chosen in order to arrive at adesired metallization composition of the Zn_(Z)Al_(1-Z) type.

FIG. 3 shows one of the spray guns 11 mounted on the oscillating plate10 of the plant as well as the wire store 20 and the associatedunwinding device or unwinder 21.

The metallization wire 12 is positioned in coil form inside a drum 22,for example a drum of the standard type for oil products, provided witha central core 23 on which the wire coil (not shown) is treated insidethe drum.

Alongside the drum there is arranged a frame 25 comprising a bracket 26which supports a wire straightening device 27 with four rollers withwhich there is associated an entry cone 28 for the wire 12, directedtowards the drum 22.

At the exit of the wire straightening device 27 there is arranged a setof wheels, a first wheel 30 of which is mounted on a slanting support 31fixed at a point 32 of the vertical upright of the bracket 26. A secondwheel 34 is mounted oscillating about a point close to the point 32 forfixing of the first wheel 30 by means of a movable support or bracket 35from which a weight 36 is suspended.

The weight 36 is fixed to a rod 37 hinged on the bracket 35.

A third wheel 38 for redirecting the wire towards the plate, arrangedabove the oscillating plate 10, is fixed to the structure of the plant,not shown.

At the exit of the third wheel 38 there is provided a second wirestraigthened device with four rollers 40.

The gun 11 has mounted at its rear end, opposite to its spray nozzle 42,a fourth wheel 44 or wheel for admitting wire into the gun, mounted on apivoting support and allowing modification of the inclination of themetallization wire 12 according to the oscillations of the plate 10.

The metallization wire 12 is removed from the storage coil containedinside the drum 22, passes through the entry neck 28, is straightened afirst time inside the straightening device 27, passes over the firstwheel 30, then underneath the second wheel 34, and then again over thethird wheel 38. It is kept tensioned by the weight 36 acting on theoscillating bracket 35 supporting the second wheel 34.

It is again straightened by the second wire straightening device withfour rollers 40 and enters into the flame gun 11 after passing over thefourth wheel 44.

The gun 11 has a device for driving the wire so as to bring it into themelting zone.

This device known per se and not shown comprises a servomotor with anincorporated tacho generator, a transmission assembly with reducinggears and pulleys for driving the wire and a device for pneumaticclamping of the pulleys, which, by means of a piston, ensures clampingof the wire between them.

As can be seen more clearly in FIG. 4, the flame gun 11 which has a base46 fixed to the plate 10 is provided at its end opposite to themetallization nozzle 42 with a cradle 48 comprising bearings 49, 50 onwhich a fork member 51 supporting the fourth admission wheel 44 ismounted in an oscillating manner.

The bearings 49 and 50 are oriented radially with respect to theoscillating plate 10.

A guide 52 for the metallization wire 12 at the entry of the wheel 44 ismoreover mounted on the fork member 51.

The bearing 50 of the cradle 48 which is located closest to the meltingzone of the gun 42 has an axial passage 56 through which the wire 12passes.

The gun has, moreover, connection sockets 58, 60 intended to receivehoses, not shown, supplying the gun with gas and oxygen.

The gas used may advantageously be propane, acetylene or natural gas.

The ZnAl alloy of the wire is therefore conveyed towards the meltingzone of the spray gun 11 so as to be melted there into fine droplets andsprayed.

The pipe T to be metallized being at the right working temperature, i.e.at about 800° C. and free from surface oxides owing to the nitrogenatmosphere which surrounds it in the metallization zone, receives jets13 emitted by the guns 11, consisting of a mist of ZnAl droplets whichadhere thereto.

As a result of the translational movement of the pipe together with thealternating rotational movement of the guns 11, it is possible to ensurea good uniformity in the thickness of the coating 62 obtained.

The alloy remains liquid on the pipe, but does not run owing to acapillary effect and also because of the very rapid surface oxidationand surface solidification of the external coating thus obtained.

During the entire cooling phase which follows the metallization phaseand which lasts about 15 minutes, the alloy reacts with the cast iron ofthe material which forms the pipe, so as to form intermetalliccompositions of the type Fe_(x), Al(_(1-x)) charged with a smallproportion of interstitial Zn.

The result obtained is a pipe coated with an external coating which isformed such that it is continuous and perfectly adherent.

The suction system associated with the enclosure and connected inparticular to the suction ducts 8, provided in the bottom 7, and suctionducts 15, provided in the upper wall 14 of the said enclosure, allowrecovery of the alloy droplets which have not reached the pipe.

By way of example a certain amount of numerical data relating to theoperating parameters and to the nature of the metallization alloy usedis given below.

The feed speed of the wire 12 is 3 m/min., its diameter is 4 mm and itslinear mass is 70 g/m.

The production yield is of the order of 50%.

As mentioned above, the wire is made either in the form of a Zn₈₅Al₁₅alloy or in the form of a lined wire with a Zn core, making it possibleto obtain a Zn₄₅Al₅₅ alloy.

The angle of rotation of the spray guns 11 is 95°.

The pitch of the spraying cycle is 70 mm.

In the tests carried out it has been possible to obtain a coating of 100g/m² to 500 g/m² with a Zn₈₅ and Al₁₅ alloy.

As shown in FIG. 5, the micrograph of a section through the coatingobtained clearly shows the formation of a well-distributed interface.

In this figure, in fact, it can be seen that a well-distributedintermetallic interface 67 has formed between the cast-iron wall 65 andthe ZnAl coating layer 66.

During the impact tests carried out on the coated pipe it was possibleto reach values of up to 150 J, i.e. up to deformation of the pipewithout deterioration of the coating.

The corrosion characteristics of the coating obtained are on a par withthose which could be obtained by means of customized hot galvanizationon a same substrate and with the same alloy.

During coating, the pipe is not subject to any annealing effect.

The metallization process according to the invention has the followingadvantages with respect to the state of the art.

It allows the thickness of the metallization coating to be controlled byadjustment of the feed speed of the wires of metallization alloy.

It enables a continuous linear coating to be obtained.

It enables a well-distributed anti-corrosion coating to be obtained.

It allows easy application of a sealant owing to the roughness conferredto the initially smooth pipe by the metallization coating.

Finally it allows lined wire as well as other alloys to be used.

What is claimed is:
 1. Process for metallization of a metal pipeobtained by vertical casting and displaced vertically in a continuousmanner with an upward movement, comprising the steps of: cooling thepipe in an atmosphere inert with respect to oxidation to a temperatureof 700 to 900° C.; and spraying onto the cooled pipe during thecontinuous, non-rotational vertical ascent thereof, a zinc-basedmetallization alloy with a set of spray guns arranged so as to surroundthe path of the pipe to be metallized.
 2. Process according to claim 1,wherein the zinc-based alloy is Zn_(x)Al_(1-x).
 3. Process according toclaim 1, wherein the metallization alloy is in the form of a wire. 4.Process according to claim 3, wherein the said wire is a wire of analloy comprising between 5 and 15% of Zn.
 5. Process according to claim3, wherein the said wire is a wire of Zn₈₅Al₁₅.
 6. Process according toclaim 3, wherein the said wire is a lined wire consisting of a Zn coresurrounded by an Al sheathing allowing a Zn₄₅Al₅₅ alloy to be obtainedafter melting.
 7. Process according to claim 3, wherein the said wire isa lined wire consisting of an Al core surrounded by a Zn sheathing. 8.Plant for metallization of a metal pipe obtained by upward verticalcontinuous casting and displaced vertically in a continuous manner,comprising, along the path of the pipe, a metallization stationcomprising a passage for the pipe, a metallization enclosure (6), sprayguns (11) mounted inside said enclosure on an oscillating plate (10)surrounding the path, means (20, 21) for supplying each gun withmetallization material (12), and means for oscillating the support plateand for attendantly displacing the guns inside the enclosure with anoscillating movement, wherein the frequency and angular movement ofoscillation is selected to ensure the formation of a coating of uniformthickness.
 9. Plant according to claim 8, wherein the passage for thepipe (T) to be coated comprises a lower part (3), a water-cooled upperpart (4), a sliding mask (5) for controlling cooling of the pipe (T)being arranged at the joint between the lower part (3) and upper part(4), the metallization enclosure (6) being arranged above the said upperpart (4) and having mounted on it a water-cooled casing (16).
 10. Plantaccording to claim 8, wherein the spray guns (11) are three in numberand are arranged on the plate at 120° from one another.
 11. Plantaccording to claim 8, wherein the spray guns (11) are flame guns or arcguns.
 12. Plant according to claim 8, wherein the metallization material(12) is a wire of ZnAl alloy and the means for supplying metallizationmaterial comprise for each gun a wire store (20) and an unwinder (21)operated by the oscillation movement of the plate (10) supporting theguns provided with means for driving the wire.
 13. Plant according toclaim 8, wherein the metallization enclosure (6) has a bottom (7)inclined downwards, from the centre towards the periphery and providedwith suction orifices (8), the said bottom (7) being connected at itscentre to a drop prevention cover (9) through which the pipe (T) to becoated passes with a minimum amount of play in order to prevent thesprayed metallization product which does not adhere to the pipe wallfrom falling towards the bottom of the plant.
 14. Plant according toclaim 8, wherein the means for supplying each gun (11) withmetallization product (12) comprise a wire store (20) and a device forunwinding the said wire from the store and conveying it towards thecorresponding gun (11).
 15. Plant according to claim 14, wherein thewire store associated with each gun (11) comprises a drum (22) forreceiving a coil of metallization wire provided with a central core (23)onto which the coil of wire is threaded.
 16. Plant according to claim 14wherein the device for unwinding the metallization wire comprises a setof wheels (30,34,38) for conveying the wire (12) from the store (20) toabove the oscillating plate (10) and a wheel (44) for admitting into thecorresponding gun (11) the wire supplied by the set of wheels(30,34,38), the said admission wheel being mounted oscillating on thegun in a radial direction of the said plate (10), its orientationfollowing the oscillations of the said plate (10).
 17. Plant accordingto claim 16, wherein the set of wheels (30,34,38) comprises a firstwheel (30) supporting the wire (12) and mounted on a fixed support (31),a second wheel (34) passing underneath the wire (12) and mounted on anoscillating support (35) with which means (36,37) for tensioning thewire (12) are associated and a third wheel (38) for redirecting the wire(12) towards the wheel (44) for admitting the wire into the gun (11).18. Plant according to claim 17, wherein the unwinding device has,moreover, between the drum (22) and the first wheel (30), an entry cone(28) for the wire (12) and a first wire straightening device (27) and,at the exit of the third wheel (38), a second wire straightening device(40).
 19. Plant according to claim 17, wherein the means for tensioningthe wire (12) comprise a weight (37) suspended from the oscillatingsupport (35) of the second wheel (34).
 20. Plant according to claim 16,wherein the wheel (44) for admitting the wire (12) into the gun (11) ismounted oscillating inside a cradle (48) of the latter by means of afork member (51) and bearings (49,50) arranged radially with respect tothe oscillating plate (10), the bearing (50) closest to the melting zoneof the gun (11) comprising a passage (56) through which the wire (12)passes.
 21. Metal pipe obtained by vertical casting, wherein it has ametallization coating obtained by the process according to claim
 1. 22.Process according to claim 1, further comprising oscillating the sprayguns about the path of the pipe.